1. Field of the Invention
The present invention relates to semiconductor devices and method of manufacturing the same. More specifically, the present invention relates to a flash memory device and method of manufacturing the same.
2. Discussion of Related Art
Generally, as the size of a NAND flash memory device shrinks, a depth of an isolation film in a cell region and a depth of an isolation film in a peripheral region in an isolation film formation process have to be different from each other.
At a boundary portion of the cell region and the peripheral region, the isolation film has a dual depth. In a NAND flash memory, in the case where an isolation film, having a dual depth at a well boundary portion, is formed, the following problems are generated.
First, when forming an isolation film necessary for each of a cell region and a peripheral region, a semiconductor substrate is etched in a dual manner, causing damage to the semiconductor substrate. Thus, there is a problem in that a leakage current path is formed to increase the leakage current.
Second, in a process of forming an isolation film required in each of the cell region and the peripheral region, a dummy active region is required. The dummy active region is formed at a well region boundary portion of the cell region or within a deep N well. In an erase operation of a device, however, a parasitic transistor is formed in the dummy active region according to a bias state. This becomes problematic in reducing a threshold voltage of the device.
Accordingly, there is a need for technologies capable of solving problems generated by an isolation film having a dual depth at a boundary portion of a cell region and a peripheral region is formed.